User:Neurogeek/Son Sound Translation

The Sound article on the French wikipedia was a featured article. It is considerably better-organized and better-written than its English counterpart. Below is my translation of Son (physique).

Sound (physicics)

An article in Wikipedia, the free encyclopedia. Go to Navigation, Search For other uses, see “Son” (disambiguation page)

Sound is a wave produced by the mechanical vibration of a support fluid or solid and propagated by the motion of the support media in the form of longitudinal waves. In physiology, sound describes the auditory sense to which this vibration is responsible for giving rise.

The science of sound is called acoustics. Psychoacoustics combines acoustics with physiciology and psychology, to determine how sound is percieved and interpreted by the brain.

Sound propagation

In a compressible medium, most commonly in air, sound is propagated by the formation of pressure gradients created by the sound source. A loudspeaker, for example, uses this mechanism. Note that only the gradients are displaced, and not air molecules, these are only displaced by micrometers. When one sees waves in water, the waves move but the water stays in the same place. It doesn't do anything but move vertically and not following along with the waves (a boat placed on the water rests in the same spot without moving). Sound propagates in the same way in solids, in the form of vibrations of atoms called phonons. There too, only the vibration propagates, and not the atoms that vibrate a tiny amount from their resting position.

The speed of sound (one could call it velocity) depends on the substance, temperature, and pressure. Because air is almost an ideal gas, pressure has very little influence on the speed of sound. In an ideal gas, velocity is given by the equations:

Therefore, the velocity of sound decreases as the density of the medium increases (effect of inertia) and as the gas's compressibility (its tendency to change volume under pressure) increases. When it travels through the atmosphere, it is helpful to know that the thermal structure of air masses affects the direction of the sound as follows: Sound propagates more poorly horizontally than at an angle because of the change in density. (This principle was used to good effect in the construction of ampitheaters since antiquity.) Attenuation is obviously weaker in the wind. (More so its reign in the sun is not too troubled. ?!) Sound can literally be carried by a low temperature inversion. For example, when temperatures fall at dusk, it is possible to hear a train at 5 km down the track without wind, and in the absence of obstacles. Sound is in this way naturally propagated along the inversion and effectually guides the wave.

Sound waves travel at about 344 meters per second in air at 20 degrees C, a speed at which one can travel at around one kilometer every three seconds, which is useful for marking a rough measure of the distance of a lightning strike (the speed of light renders the perception of lightning almost instantaneous.) In solids (non-gasses) sound propagates much faster. Thus, in water its speed is 1482 meters per second, and in steel it is 5050 m/s. Sound cannot propagate through a vacuum, because there is no matter to support the waves ( sound isolation), sound propagates dependent on the displacement of molecules of air. It is a wave said to be longitudinal, because the component molecules displace in the same direction as the wave (the other type are transverse waves).

Frequency and Pitch

The frequency of sound is expresses in Hertz (Hz, it is directly linked to the pitch of the sound percieved but not for complex sound waves (see the article on Psychoacoustics). A low frequency corresponds to a base sound, a high frequency a treble.

Every living thing endowed with a sense of hearing can only perceive a part of the sound spectrum:

Physiologists agree that the human ear hears those sounds best which are common in their environment (depending on the age, culture, etc.) between 20 Hz (lower qualifies as infrasound) and 20 kHz (above qualifies as ultrasound). Cats can perceive sounds to 25 kHz Dogs can perceive sounds to 35 kHz Bats and dolphins can perceive sounds at 100 kHz.

Certain animals use their ability to cover a large range of frequences for diverse purposes:

Elephants use infrasound to communicate an many kilometers distance Dolphins communicate using ultrasound Bats emit ultrasounds (~80 kHz) with their echolocation system which permits them to move in total darkness.

To have frequencies correspond to the musical pitches of the tempered scale (?) (classical western music) see “tempered scale” > comparison of three systems of division of the octave.

Amplitude and intensity

Unit of measure

Elsewhere, or typically pressure is measured in pascals, but in acoustics intensity is measured in decibels (dB). It is a unit which uses the logarithm of the ratio of the sound intensity with a reference intensity, expressed in watts per square meter (Wo = 10-12Wm2). This method was chosen to allow easily manipulated numbers, which do not become too large or small (see the article Logarithmic scale), and because this approach corresponds well to the perception of sounds in terms of the sound sensation.

But be warned, the idea of sound levels only gives a vague idea of the perceived sound. For that, it is necessary to take into account the shape of the ear, which varies in its sensitivity depending on the sound frequency (the ear is less sensitive to bass frequencies). A better approximation of perceived volume is given in audible decibels (dBA), which can be measured electronically after filtering the signal by a “balance filter A” (there also exist balance filters B and C adapted to measure sounds of greater intensity). Zero dB corresponds to the monimum that the human ear can perceive, called the audible threshold, and does not correspond to absolute silence. Cit values was chosen experimentally for a sound at the frequency of 1000 hZ, which is 10-12 W m2. Most people have a auditory threshold above 0 dB (approximately 4 dB). The threshold of pain is 120 dB, but the ear can suffer damage at 85 dB (see the article on Psychoacoustics).

It is enough to change the reference of force or pressure (Po or Wo in the formulas here) because the scale of volumes is completely changed. This is why the decibles printed on the volume control of a Hifi sterio to correspond at all to the acoustic levels but to electronic pulses run to the amplifier, which is easily seen: the value of 0 dB represents there the maximum pressure that the amplifier is capable of delivering.

Sound level in force – Sound level in pressure

Different measures of amplitude

There are many methods of measureing the amplitude of sound, and by extension, of any sinusoidal signal: The average amplitude (the mean value of the positive signal) The effective amplitude (continuous amplitude equivalent in pressure) The peak amplitude (maximum positive) The crest-to-crest amplitude (the maximal distance between positive and negative amplitude)

Practically, the average amplitude is of little interest and is not used. In contrast, the effective amplitude or RMS (Root Mean Square) ,which is the square root of the average of the signal, is universally adopted to measure the value of alternating electric current, in the general cast also and be used in acoustics. A speaker which is rate for 10 watts RMS makes 14 watts at the peak and 28 watts crest-to-crest (also written cc). The measures of pressure crest-to-crest also can be called “musical watts” by vendors of audiovisual materials because the numbers are more flattering.

Timbre

Detailed article: Timbre (music)

This is the characteristic which identifies a sound in a unique fashion. Two sounds can have the same fundamental frequency and the same intensity; but have very different timbres.

Space-time

As in all perceived phenomena, time plays a fundamental role in acoustics (and even more in music). There exists several relationships very strict between space and time, which one can see in that the sounds is a wave which propagates in space over the course of time.

There are three broad classes of acoustic signals: Periodic, which repeat identically over time Irregular, which do not have periodic characteristics. Dans ce qui suit, and in a general manner, one is only interested in a limited group of these signals;p those which have some stable statistical characteristics over time. These are called irregular ergodique signals. Concretely, this type of grey or white noise is used by scientists and certain artists. Impulsive: which don't repeat in time and have a specific form.

All these signals can be defined and analyzed equally in the time-space or in the frequency-space. In the latter, one has recourse to use the signal spectrum, calculated by its frequency definition (called a Fourier domain). The spectrum of a signal represents the different “notes” or pure tones contained in a sound, called partials. In the case of a stable periodic signal such as a siren, the spectrum does not change over the source of time and presents a single value called “raie”. It is in effect possible to consider every sound as the combination of a group of “pure ones” which are sinusoids (see on this subject the article on Fourier transforms).

Register

Detailed article: Sound register

In music, the register of a piece of music has place for keeping the track.

Music

Sound and computers

Recording

For electronic sound recording (recording on a computer), it is necessary to use an algorithm to convert analog sounds to digital, which one calls acquisition. This operation consists of transforming the variations of sound pressure, into numbers which can be better digitally analyzed. This transformation is called sampling. A microphone converts the variations of pressure in the air into electronic signals which has an analog-to-digital converter (ADC) which can sample the signal at regular intervals, and transform it into a digital record. This work at the present is accomplished by sound cards in personal computers.